Rubbery copolymers containing a majority of isobutylene units are well known for their low gas permeability, unique damping properties, and low surface energy that make them particularly desired in applications such as inner tubes and tire innerliners. For better compatibility or co-curability with other elastomer components in the applications, an unsaturated comonomer and/or a comonomer containing reactive functionality has been used. Among the isobutylene polymers, the isobutylene/para-methylstyrene copolymers (IPMS) are of particular interest. The para-methylstyrene (PMS) derived units in the polymers can be partially brominated to give an isobutylene/PMS/BrPMS terpolymer (BIMS). The BIMS can be further functionalized via the reactive benzylic bromine for conversion to a variety of functionalized isobutylene polymers, as described in U.S. Pat. No. 5,162,445. Another advantage of IPMS copolymers and BIMS terpolymers is their excellent resistance to ozone and aging due to the completely saturated backbones.
The tire industry has a desire to enhance the barrier property of elastomers used in inner tubes and innerliners. For example, elastomer nanocomposites have been developed. Nanocomposites are polymer systems containing inorganic particles with at least one dimension in the nanometer range. Some examples of these are disclosed in U.S. Pat. Nos. 6,060,549, 6,103,817, 6,034,164, 5,973,053, 5,936,023, 5,883,173, 5,807,629, 5,665,183, 5,576,373, and U.S. Pat. No. 5,576,372. Common types of inorganic particles used in nanocomposites are phyllosilicates, an inorganic substance from the general class of so called “nanoclays.” Ideally, intercalation should take place in the nanocomposite, wherein the polymer inserts into the space or gallery between the clay surfaces. Ultimately, it is desirable to have exfoliation, wherein the polymer is fully dispersed with the individual nanometer-size clay platelets.
Unfortunately the incompatibility between the hydrophobic isobutylene elastomer and hydrophilic inorganic clays has made it very difficult to achieve a good clay dispersion or exfoliation in the elastomer. One approach has been the use of organically modified montmorillonite clays. Organoclays are typically produced through ion-exchange reactions that replace sodium ions that exist on the surface of sodium montmorillonite with organic molecules, such as alkyl or aryl ammonium compounds known in the industry as swelling or exfoliating agents. See, e.g., U.S. Pat. No. 5,807,629, WO 02/100935, and WO 02/100936. Other background references include U.S. Pat. Nos. 3,516,959, 3,898,253, 5,333,662, 5,576,373, 5,633,321, 5,665,183, 5,807,629, 5,936,023, 6,036,765, 6,121,361, 6,552,108, WO 94/22680, WO 01/85831, and WO 04/058874.
Functionalization of the BIMS polymers for use in nanocomposites has also been shown to provide a better interaction between the functionality on the polymer and clay surface, which can lead to a higher degree of clay dispersion and exfoliation. This, in turn, can provide the nanocomposite with an even better barrier property. The preferred functionalities for permeability improvements in BIMS polymers have been ammonium (—NR), hydroxyl (—OH), ester (—OOR), and ether (—OR).
Unfortunately, when ammonium functionality is incorporated into a polymer and/or a nanocomposite with clay, the viscosity of the polymer can increase significantly due to the ionomeric associations of the functional groups in the polymer backbone. A low viscosity is needed to facilitate processing of the elastomer in conventional rubber compounding and tire building equipment. One way to try to attain a low viscosity has been to include a higher alkyl tail in the functional group, which can inhibit the ionomeric interactions. For example, published applications US 2005/0027057, US 2005/0027058 and US 2005/0032937 disclose treatment of BIMS polymers with tertiary amines preferably having a long chain alkyl substituent.
The tire industry has a continuing need for elastomers and nanocomposites that can be used in air barrier applications, having both an improved barrier property and a controllable processability.